It's probably going to be fairly difficult to find a female voice actress. Why not make an artistic choice to have Eris as a silent character, with the other actors reacting as though she had actually spoken?
If you can find a female VA, by all means - shoot the lines. I feel like you might be able to make Eris more relatable by having her be silent - especially while her fireteam is obliterated and Toland learns the song.
[spoiler]*giraffe noise*[/spoiler]
English
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Besides, Eris does talk in the Grimoire card where Eriana interrogates the Wizard and learns about about Crota. That scene is in the film, so it would also contradict the grimoire, which is one thing we can't do. It's an interesting idea but we're too far along now to use it.
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You're right, it is going to be hard to find a female voice actor, but given that we've already filmed all of the scenes and have the script ready to go, making Eris silent would require weeks, if not months of reshoots and rewrites. It's not a bad idea, but it would probably be too much of a hassle to implement.